Sunday 25 October 2015

Techniques on Titration

Titration is a method of analysis that allows you to determine the endpoint of a reaction. To perform a titration accurately, a precise quantity of the titrant needs to be dispensed into the reaction flask

ErlenmeyerIn a titration, the reaction flask is usually an Erlenmeyer flask. It is used because it is conical in shape. This shape makes it easy to swirl the flask without spilling. For example, in an acid-base reaction, you may choose to pipette the acid into the Erlenmeyer flask. The base is introduced to the acid with the use of a burette. The solution in the burette, in this case, the base, is known as the titrant. A colour indicator such as phenolphthalein is introduced into the Erlenmeyer flask to detect the endpoint of the reaction. In acidic solution, phenolphthalein is clear in colour. In basic solution, phenolphthalein turns pink. The endpoint of the reaction is reached when the solution turns pink and the pink colour does not disappear after 30 seconds.

Steps in performing a titration;
1. Follow the steps in Techniques on the Use of a Burette

2. Take an initial volume burette reading and enter it on the datasheet.

3. Follow the steps in Techniques on the Use of a Pipette  and pipette the acid into the Erlenmeyer flask (figure 2).

Pipette the acid into erlenmeyer

4. Set up the burette and Erlenmeyer flask such that the tip of the burette is inside the neck of the Erlenmeyer flask (figure 3). This ensures that all the base will be dispensed into the Erlenmeyer flask.

5. Add a few drops of colour indicator, such as phenolphthalein, to the acid solution.
erlenmeyerAt the start of the titration, the acid solution in the Erlenmeyer flask should be clear. Add the titrant to the titration flask slowly and swirl the flask frequently. When the titrant touches the acid solution, the solution briefly turns pink in colour. Upon swirling, the pink colour will go away. Slow down the addition of the titrant when the trail of pink colour is taking longer to go away. Reduce the volume of the additions as the titration progresses. When you are near the endpoint, the titrant should be added a drop at a time.

6. When it is judged that only a few more drops are needed, rinse down the walls of the Erlenmeyer flask. Quickly spin the closed stopcock 180 degree. This allows a small
shot of titrant to shoot out.

7. When the volume of titrant to be added is judged to be less than one drop, open the stopcock so that only part of a drop appears. Close the stopcock and touch the drop on the side of the Erlenmeyer. Use the wash bottle to rinse the partial drop into the Erlenmeyer flask with swirling.

8. The endpoint is reached when the colour change does not disappear after 30 seconds. The phenolphthalein colour change is from clear to pale pink (figure 4).
The color change

9. Read the final burette volume. The difference between the initial and final readings on the burette is the volume of base used in the titration.

10. Repeat a titration at least twice. The burette volumes should be within +/- 0.10 mL or less.

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